<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box {
      width: 150px;
      height: 150px;
      border: 1px solid red;
      /* #3 */
      display: flex;
      /* #4 */
      flex-wrap: wrap;
      /* #6 */
      align-content: space-between;
    }

    .column {
      /* #5 */
      flex-basis: 100%;
      /* #1 */
      display: flex;
      /* #2 */
      justify-content: space-between;
    }

    .column span {
      width: 50px;
      height: 50px;
      background-color: pink;
      border-radius: 50%;
    }
  </style>
</head>

<body>
  <div class="box">
    <div class="column">
      <span class="item"></span>
      <span class="item"></span>
    </div>
    <div class="column">
      <span class="item"></span>
      <span class="item"></span>
    </div>
  </div>
</body>

</html>